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Sheet metal workers

Sheet metal workers fabricate, install and maintain products from large metal sheets : roofs, sidings, air ducts, gutters, even outdoor signs are made by these highly skilled craftspersons.

Following detailed plans they cut, mold, bend and shape large pieces for installation at a job site.

Sheet metal workers should have knowledge of drafting, reading blueprints, handling tools and welding. The job also requires standing for long periods, bending, squatting and lifting heavy materials. Some workers pick up the trade as helpers who carry metal and clear debris. However formal apprenticeship is the best way to learn the craft. Local chapters of professional associations run these programs. To qualify applicants need a high school diploma or equivalent. Unlike many other construction trades very few sheet metal workers are self-employed. People who are willing to invest the time to master this valuable craft can become respected specialty craftspersons or even the owners of their own business.

 

10. Turning ore to steel

Turning low-grade ore into the most used metal in the world starts in a 200-foot tall furnace. This is where the iron ore is heated with heat generated by a type of fuel called coke. Coke is a result of burning the impurities in the junk out of coal you don’t need to get to the good stuff which is going to burn cleanly and give you the nice part of clean fire that you need to make steel. It is called the blast furnace because as the ore and coke flow downward they are met by a 16 hundred degree blast of hot air moving at 110 thousand cubic feet per minute. When this is coming to what we call the bosh level of the furnace to nozzles which are called tuyeres (and we have 18 of them depending of the size of the blast furnace) the air is pushing up through a solid burden material. The superheated air ignites coke like charcoal in a grill and heats up the furnace to nearly 4000 degrees Fahrenheit. At this temperature the oxygen is finally blasted away leaving pure molten iron. But it’s also doing something even more amazing. As the heat from the coke flame strips away the oxygen, the carbon in the coke is bonding to the iron.

Under the extreme heat the mixture liquefies, impurities like silica and sulfur rise to the top, the heavier iron sinks to the bottom where it’s thrown out in a glowing molten stream. Every 45 minutes some 500 tons of molten metal called pig iron catches from the tap. It’s called pig iron because it was originally cast into moulds resembling piglets suckling a sow. But ironically till the iron is freed from the oxygen, the molten pig iron that flows from the furnace has a new problem: too much carbon - over 4% which makes the resulting metal much too brittle to become steel That number needs to be reduced to less than 2%.

 

11. Steel heat treating

To understand the benefits of heat treating processes first requires an awareness of metal and alloy structures.

When a molten metal solidifies the atoms arrange themselves into definite patterns called crystal structures.



The two most common crystal structures in metals are body-centered cubic and face-centered cubic.

These crystal structures grow uniformely in all directions within each developing crystal.

As the metal cools these crystals are confined by the adjacent developing crystals forming grains.

The line of intersection between grains is called a grain boundary.

Because the grains form independently their crystal structures develop tilted in various directions.

All atoms in this crystalline structures are held in place by electro-magnetic attraction to neighbouring atoms.

If a force or a load is applied to a metal these electromagnetic bonds stretch allowing the atoms to move slightly.

When the load is removed the bonds pull the atoms back into position.

If the applied force exceeds the metal yield strength those electro-magnetic bonds will break causing permanent stretching or deformation.

To make metals stronger and more resistant to deformation it’s necessary to strengthen their crystal structures.

This is done by adding alloying elements which are other metals or non-metal elements like carbon.

The addition of an alloy introduces foreign atoms within crystal structure of the base metal disrupting the structural uniformity.

This disruption results in an increased strength.

 

12. Ductility of metals

Iron is malleable at high temperatures. Therefore, it must be heated up before forging. When it is red hot it can be pounded with a hammer into a desired shape. Likewise, two pieces of red-hot iron can be linked together to form part of an ornamental trellis for example. Copper and aluminium are malleable at room temperature. Metals can be drawn into rods and wires by heating and pulling apart. Copper can be drawn into the very thin wires used in electric cables.

 

13. Brittle and ductile materials.

I am a geologist who studies how rock deform.

So what am I doing in my kitchen ? I am in my kitchen because rocks deform in the same way a lot of other things deform.

When something changes its shape, it responses to stress. It’s being deformed. Deformation can be brittle or ductile. Take a bread stick. Bend it. It breaks. It’s brittle deformation. Take a caramel bar. Bend it. It changes shape but it stays a one piece. That’s ductile deformation.

What controls whether something is brittle or ductile ? Three things: temperature, composition and something called “strain rate”.

Let’s consider how temperature affects whether something is brittle or ductile.

Take a frozen stick of butter. Bend it. It breaks. That’s brittle deformation.

Now take butter at room temperature. It flows like the caramel bar. That’s ductile deformation.

Now if we take a candle at room temperature , bend it – it breaks. That’s brittle. But what happens if we warm it up a bit ? If the candle is warmed up a bit, it flows like the caramel bar. Butter at room temperature and wax at room temperature deform differently because they have different compositions. But at lower temperatures both are brittle. And at higher temperatures both are ductile.

So what do butter and wax have to do with rocks ? Rocks are cold near the earth surface and they get hotter and hotter with depth. Cold rocks near the surface tend to break if they are stressed. They are brittle. At about 15 kilometers within the earth most most rocks are hot enough to flow if they are stressed. They are ductile.

Let’s talk about strain rate (which is how fast deformation occurs). Different strain rates can cause the rocks to be either ductile or brittle at the same temperature. Here’s how.

Take Silly Putty (*). If I pull it slowly it flows in a ductile fashion. I pull it fast. It breaks. It’s brittle. Layers of sedimentary rock are just like these layers of play-doh (**). If I push on them slowly, they fold. Some mountains made of folded rocks look just like this.

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* Silly Putty (also known as Thinking Putty) is a toy based on silicon polymers. It bounces, but breaks when given a sharp blow; it can also flow like a liquid and will form a puddle given enough time

 

** Play-Doh is a modeling compound used by young children for art and craft projects at home and in school. Composed of flour, water, salt, boric acid, and mineral oil.

 


Date: 2015-02-03; view: 911


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